1. Field of the Invention
This invention relates to a reader/writer apparatus that is equipped with an antenna and communicates with an identifier such as an IC tag, particularly to a reader/writer that is equipped with an antenna that achieves a good communication state.
2. Description of the Prior Art
Contactless reader/writer systems using IC tags are commonly called RF-ID systems, and are applied in transportation systems, distribution management systems, inventory management systems, and so forth.
The main components of these systems are an IC tag equipped with an IC chip and loop antenna, and a reader/writer apparatus that is equipped with a loop antenna and performs wireless communications with the IC tag. By means of its loop antenna, the reader/writer apparatus transmits, continuously or intermittently, electric power to drive the IC chip of the IC tag and interrogation data, and obtains response data from an IC tag that is within receiving range of the electric power and interrogation data.
An RF-ID system will now be described, using as an example an electromagnetic induction type reader/writer apparatus that uses the 13.56 MHz frequency band.
For a contactless IC tag that does not contain a power supply to wirelessly communicate with an electromagnetic induction type reader/writer apparatus, the magnetic field generated by the transmission antenna in the reader/writer apparatus has to be spatially within a range in which it can generate the electromotive force to drive the circuit inside the IC tag and, in addition, has to have a positional relationship whereby it has a directional interlinkage to the loop plane of the loop antenna of the IC tag.
However, if it is assumed that, with respect to the loop antenna of the reader/writer apparatus, the IC tag is within an arbitrary, uniform planar or spatial read range that is adequately within the necessary distance for driving the IC tag, and the IC tag plane (loop antenna plane) is fixed in an arbitrary direction, the loop antenna of the reader/writer apparatus has one or more null points at which, with respect to the IC tag, the magnetic field disappears. Moreover, continuous null points form a linear, planar or three-dimensional region in which communication is impossible.
FIG. 6 is an example of a loop antenna 41 of a reader/writer apparatus, showing the direction of the magnetic field generated by the loop antenna 41, the plane direction of the IC tag 42 (direction of the plane of the IC tag loop antenna), and the null plane (continuous null points) at this time.
More specifically, a book management system, such as that used in a library, will be described as an example of an inventory management system that uses loop antennas on a plurality of shelves.
Affixed to each book is an IC tag with an IC chip on which is stored book management data. Book management data can include data relating to the book itself, such as title, author, category, and data on its lending history, such as the number of times it has been lent, and the periods of the loans. Each bookshelf has an antenna used by the reader/writer apparatus to communicate with the IC tag affixed to each book. The antennas of the IC tags and reader/writer apparatus are directional and are disposed for mutual suitability between IC tag antennas and shelf antennas. The system can be used to provide users with various services, such as searching for books on specific shelves, or searching for the location of a specific book, and so forth.
As long as there is mutual suitability between the directionality of the shelf antennas and the directionality of the IC tags on books arranged arbitrarily on the shelves, the read range is arranged for uniform read capability.
FIG. 7 shows an examplary configuration of a book management system comprising a bookcase 51, a bookshelf 52, a loop antenna (shelf antenna) 53 of a reader/writer apparatus on the bookshelf 52, and a book 54 with an IC tag 55. FIG. 7 also shows the direction of the magnetic field generated by the loop antenna 53, and the read range (in the plane of the loop antenna 53). When the IC tag 55 is affixed to the cover of the book 54 and the book 54 is arranged normally on the bookshelf 52, the direction of the magnetic field interlinking the IC tag 55 is the lateral direction of the bookshelf 52.
With the conventional square-shaped shelf antenna pattern, the range of the magnetic field generated in the desired lateral direction of the shelf is a limited portion of the side surface of the bookshelf, so it is not possible to read to the inside of the shelf antenna pattern, and therefore not possible to obtain a uniform read range on the shelf.
FIG. 8 shows an example of a loop antenna 53 used as an ordinary shelf antenna, the plane direction of an IC tag 55, and the ranges in which reading is possible and not possible with the loop antenna 53. Moreover, if the output of the reader/writer apparatus is increased in order to expand the read range, communication can be obstructed by shelf antennas above and below, or by other reader/writer apparatuses being operated adjacently or nearby. Also, in cases in which the intention is to read a plurality of IC tags in a shelf antenna all at once, it is difficult to uniformly form on the shelf a magnetic field having the necessary strength and directionality to read the information of all the IC tags.
Reference 1: Unexamined Patent Application Publication 2004-140513.
Thus, a problem with conventional book management systems and the like is that it is difficult to achieve a state of good communication between the reader/writer apparatus antenna and the IC tags. Therefore, when the IC tag 55 is fixedly oriented in the lateral direction of the bookshelf 52, as shown in FIG. 9, there is a need to be able to obtain a desired uniform read range over the whole surface of the bookshelf 52 by forming a magnetic field of uniform directionality and strength over the whole surface of the bookshelf 52. Even if the magnetic field is not as shown in FIG. 9, there is a need to be able to form a good read range that covers the whole of the bookshelf 52 surface.
In order to have the strength of the magnetic field generated by the loop antenna 53 increase the nearer it is to the antenna pattern, it is desirable to make the antenna pattern one that uniformly covers the whole of the bookshelf surface, as in the case of FIGS. 10(a) and (b), which shows examples of the strength and direction of magnetic fields generated by the loop antenna 53 in each case.
Also, when, as shown in FIG. 11, an antenna 61 is used that includes a pattern folded back inside the conventional loop shape, the peaks and valleys of the folds form null planes, making it all the more necessary to develop a well-shaped antenna.
Therefore, the object of the present invention is to provide a reader/writer apparatus equipped with an antenna that achieves a good communication state with an identifier such as an IC tag.